A variable speed constant frequency aircraft power generating system is a solid state device which converts wild frequency AC power produced by a generator driven at variable speed into constant frequency AC power useful for powering electrical systems on board the aircraft. VSCF systems employ, among other things, large switching power transistors as a part of an inverter to convert DC power into constant frequency AC power. By-products of the transistors' operation are unwanted higher harmonics and electromagnetic interference ("EMI"). EMI is higher harmonics usually defined to be equal to or exceeding the 200th harmonic of the fundamental. It is desired to suppress these higher harmonics and EMI. The instant invention provides a compact, lightweight and inexpensive solution to the problem of attenuating higher harmonics and EMI produced in a VSCF system.
In typical VSCF systems, solid state components are housed within electrical chasses. These chasses act as an electrical shield, preventing EMI produced and existing within the chasses from leaving therefrom. However, electrical leads enter and exit the chasses, carrying through them the EMI, thereby defeating the shielding provided by the chasses. It is desired to provide a means in the form of an electrical passageway through the chasses in order to allow fundamental frequency electrical power to enter and exit the chasses without carrying with it the undesirable higher harmonics and EMI.
This desired function usually takes the form of a feed-through capacitor. A feed-through capacitor comprises a conductive shaft or stud that passes through a chassis wall. A capacitive element usually surrounds the stud and is coupled to the stud and the chassis wall on the inside of the chassis The capacitive element provides a low impedance path from the stud to the chassis wall for higher harmonics and EMI. The capacitive element appears as an insulator to low frequencies on the stud and therefore does not divert the low frequencies from the stud to the housing. These low frequencies are allowed to pass through the chassis wall unattenuated.
It is common to find feed-through capacitors used in low power radio frequency ("RF") applications, where the fundamental frequency is in a radio frequency range and EMI appears as interference of a frequency many times that of the fundamental. Also, in RF applications, the current at the fundamental frequency is far below one ampere. Following is a discussion of examples of RF attenuating feed-through capacitors and related devices that define the background art to which the invention to be described is an improvement.
U.S. Pat. No. 4,229,714, which issued on Oct. 21, 1980 to Yu, is directed to an RF coaxial connector assembly, having an inner and outer conductor, which employs a washer-like capacitor fitted over the outer conductor for low frequency isolation and which employs a transformer adapted for connection between a source of RF signals and the inner and outer conductors of the assembly for reducing the RF currents on the outer conductor. Reduction of the RF currents on the outer conductor is effective to reduce the radiation of RF energy from a coaxial cable coupling the connector assembly to the load. Yu shows resilient conductors supporting the capacitor on its sides. Such an arrangement provides a tortuous path for high frequencies from the outer conductor to a chassis wall.
The present invention is not a coaxial connector assembly, nor is it desired to pass RF signals through the inner conductor. In contrast, the present invention employs resilient rings on inner and outer surfaces of an annular capacitive element to allow comparatively high frequencies to be shunted to the wall of an EMI shielded chassis in as direct a path as possible. In Yu, the inner conductor is not shielded; in the present invention, it is.
U.S. Pat. No. 2,756,375, which issued on July 25, 1956 to Peck, is directed to a feed-through ceramic capacitor employing a metallic casing consisting of a top and a lower depending body portion surrounding a series of metallized ceramic disks which in turn surround a feed-through wire, the entire assembly being sealed in a fixed relationship in which the capacitor element is not subject to tension or shear.
The present invention employs a somewhat different arrangement of capacitor elements to achieve an entirely different object. While Peck is concerned with breaking the capacitive element by bending the feed-through wire, the present invention is concerned with stresses which may damage the capacitive element due to extreme operating temperatures of the device. Accordingly, resilient conductive rings are employed within the present invention to allow for these thermal stresses to be dissipated.
U.S. Pat. No.4,314,213, which issued on Feb. 2, 1982 to Wakino, is directed to an improved through-type capacitor for use in electrical and electronic equipment which includes a metallic housing having upper and lower openings through a stepped portion. A capacitor element is accommodated in the upper opening and includes a disk member with a central bore and electrodes provided on its opposite surfaces. A central conductor is inserted through the central bore of the disk member so as to be secured for being electrically connected to one of the electrodes, with the other of the electrodes being electrically connected to the stepped portion of the housing. Finally, an electrically insulating support plug extends through by the central conductor and is fixed to the lower opening, and resin material filled in the upper opening of the housing.
Again, Wakino is directed to a device for attenuation of RF range frequencies. Accordingly, Wakino fails to provide for a device which can handle high power at lower frequencies. The device in Wakino could never operate at the temperatures contemplated by the present invention.
None of the aforementioned patents is directed to providing a feed-through capacitor which is able to handle high power (power in excess of 1 ampere and, in the preferred embodiment, up to 700 amps) and to provide a direct shunt path for high frequencies. The aforementioned devices fail to contain structure necessary to alleviate thermal stress created during operation of a feed-through capacitor. Absent means for alleviating the thermal stress, the capacitive element in each of the above devices would shatter or otherwise become inoperable, rendering the feed-through capacitor useless.
The present invention is the first to address the Problem of providing a compact, lightweight feed-through capacitor which includes resilient structure necessary to overcome thermal stress to allow the feed-through capacitor to operate in a high power, high current, high temperature environment, the kind of environment encountered by variable speed constant frequency aircraft power generating systems.